This invention relates to high speed semiconductor devices and especially to field effect transistors using silicon on sapphire material and to methods for their fabrication.
Conventional monolithic integrated circuits employing field effect transistors (FETs) are fabricated on silicon substrates, with isolation between devices being accomplished by use of P-N junctions. Spacing between the source and gate and between the gate and drain is limited by the mask to substrate alignment capability associated with the photolithographic processing. A conventional field effect structure is illustrated in FIG. 1. (The source, gate, and drain regions are indicated at 10-12; conductors to those regions at 13-15; and the channel at 16.) The switching speed of the conventional field effect transistor is severely limited by the source-to-drain feed-through capacitance associated with the P-N junction isolation, and the added channel resistance resulting from the spacing between source and gate and between gate and drain.
Silicon bipolar and silicon FET hybrid circuits are good performers at L and S band and are more cost effective than the gallium arsenide FET. However, attempts to fabricate monolithic integrated microwave amplifiers using these silicon structures have been frustrated by the high parasitic capacitances associated with the isolation of elements and the high conductor to substrate capacitances.
The principal object of this invention is to provide a device structure for improved silicon microwave amplifiers and high speed switching circuits, along with simple fabrication methods.